Rail to rail operational transconductance amplifier

An operational amplifier having inputs and supplied by first and second supply voltages comprising:

• a first pair of transistor connected between the inputs and coupled to a source of the first supply voltage through a first reference current source supplying reference current,
• a second pair of transistors connected between the inputs and coupled to a source of the second supply voltage through a second reference current source supplying the reference current,
• said amplifier operating in an operating range between the first and second supply voltages consisting of a middle range between a first predetermined voltage and a second predetermined voltage, a first end range between the first supply voltage and the first predetermined voltage, and a second end range between the second supply voltage and the second predetermined voltage, said first predetermined voltage being equal to a voltage turning off the transistors of the first pair, said second predetermined voltage being equal to a voltage turning off the transistors of the second pair,
• a first controlled current generator coupled to the first reference current source and controlled through the inputs of the operational amplifier for supplying a first constant current equal to a predetermined fraction of the reference current in the middle range, said first constant current being subtracted from the reference current, said first controlled current generator being blocked in the second end range, and
• a second controlled current generator coupled to the second reference current source and controlled through the inputs of the operational amplifier for supplying a second constant current equal to the predetermined fraction of the reference current in the middle range, said second constant current being subtracted from the reference current, said second controlled current generator being blocked in the first end range.

An amplifier having inputs and supplied by first and second supply voltages comprising:

• reference current means for supplying a reference current to the amplifier,
• first transistor means connected between the inputs and coupled to a source of the first supply voltage through said reference current means,
• second transistor means connected between the inputs and coupled to a source of the second supply voltage through said reference current means,
• said amplifier operating in an operating range between the first and second supply voltages consisting of a middle range between a first predetermined voltage and a second predetermined voltage, a first end range between the first supply voltage and the first predetermined voltage, and a second end range between the second supply voltage and the second predetermined voltage, said first predetermined voltage being equal to a voltage turning off the first transistor means, said second predetermined voltage being equal to a voltage turning off the second transistor means,
• first controlled current generating means responsive to the inputs of the amplifier for supplying a first constant current equal to a predetermined fraction of the reference current in the middle range, said first constant current being subtracted from the reference current, said first controlled current generating means being blocked in the second end range, and
• second controlled current generating means responsive to the inputs of the amplifier for supplying a second constant current equal to the predetermined fraction of the reference current in the middle range, said second constant current being subtracted from the reference current, said second controlled current generating means being blocked in the first end range.

A transconductance amplifier comprising:

• a first differential amplifier with a pair of transistors of a first type, each of which is controlled by one of two inputs, the quiescent current of said transistors being fixed to a predetermined value by a first current source connected to a first supply rail;
• a second differential amplifier with a pair of transistors of a type opposite to that of the first, each of which is controlled by one of said inputs, the quiescent current of these transistors being fixed to said predetermined value by a second current source connected to a second supply rail;
• the transconductance amplifier having a first operating range where the two differential amplifiers are active and second and third operating ranges where only one differential amplifier is active;
• means for deriving a predetermined fraction of the currents provided by the current sources to the differential amplifiers, said means being active in the first operating range and inactive in one of the second or third ranges, and the predetermined fraction being such that the transconductance of the active transistors in one of the second or third ranges is equal to twice the transconductance of the transistors in the first range;
• wherein said means comprise:
• a third pair of transistors of the opposite type, each of which is controlled by one of said inputs, said transistors are arranged in parallel between the first current source and a third current source connected to the second supply rail;
• and
• a fourth pair of transistors of the first type, each of which is controlled by one of said inputs, said transistors are arranged in parallel between the second current source and a fourth current source connected to the first supply rail;
• the current provided by each of said third and fourth current sources is equal to said predetermined fraction.